IP: 802.11a: First Glimpse

[David Farber <dave () farber net>]

> Date: Wed, 29 Aug 2001 14:30:32 -0700
> From: "Robert J. Berger" <rberger () ultradevices com>
>
> 802.11a: First Glimpse
> http://www.80211-planet.com/columns/article/0,4000,1781_873181,00.html
> By Ted Stevenson
>
> It's finally in silicon. The long-awaited 54Mbps technology that's
> expected to light up the 5GHz radio spectrum is available and being
> tested by at least one chip supplier and its customers. Now, the
> lively - and hitherto theoretical - debate on the relative
> capabilities of 802.11a and 802.11b can be waged on the basis of
> real-world observation.
>
> Atheros Communications paid a visit to 802.11 Planet late last week
> and gave us a demonstration of its single-card, "reference design"
> 802.11a components, based on early production runs of the Atheros
> AR5000 chipset, which will be shipping in quantity to OEM customers
> soon. Atheros has been using the hardware, based on its two-chip
> solution, in its own internal network for some time.
>
> The demo consisted of the famous Madonna BMW video beamed from a
> notebook server to an 802.11a access point and then to another
> notebook workstation. We carried the workstation around our offices
> with some freedom within a range of 75 feet or so with no
> deterioration in quality until very substantial impediments (heavy
> concrete walls) interrupted the signal.
>
> Following the film credits, product manager James Chen discussed with
> us the company's early, in-house testing findings, at some
> length. Today, we'll look at three key issues: 802.11a's effective
> range, power consumption, and the cost of the hardware. Tomorrow,
> we'll look at some additional topics, including a "turbo" mode that
> allows data links at up to 108 Mbps!
>
> Circles within circles Pre-release popular wisdom holds that 802.11a
> should offer a much smaller radius of coverage than does 802.11b
> technology, which is generally held to be effective up to about 300
> feet or so. Theoretical calculations put 802.11a coverage at roughly
> one-fourth of that range. But in systematic empirical tests carried
> out at Atheros's offices, technicians found that 11a operates with
> acceptable reliability to well over 200 feet. Moreover, throughout
> most of its range, including the maximum, it offers a throughput
> advantage over 802.11b at the same distances.
>
> According to Chen, typical 802.11b deployments use a cell radius of
> about 65 feet (based on access points with power in the 15dBm range),
> which ensures reliable coverage at the maximum data link rate of
> 11Mbps. Beyond 100 feet or so, an 802.11b link steps down to 5.5 Mpbs,
> and at roughly 175 feet, to 2 Mbps.
>
> With 802.11a, on the other hand, Atheros testing established stepdown
> points at about 20 feet (from 54Mbps to 48Mbps), 40 feet (to 36 Mbps),
> 75 feet (to 24 Mbps), 85 feet (to 18 Mbps), 135 feet (to 12 Mbps) and
> 175 feet (to 6 Mbps).
>
> The points Chen chose to stress are that at the 65-foot "sweet spot"
> distance, 802.11a appears to hold about a 3.5x data-rate advantage,
> and even out at the 225-foot distance, it betters 802.11b by about 3
> to 1. (At certain other points in the curve, the advantage is less.)
>
> Cost questions Based on these range-performance figures, Atheros now
> calls into question another commonly held notion: that 802.11a is less
> power-efficient than its 2.4GHz cousin. The company's argument is
> based not on power consumption per time unit, but power consumption
> per megabyte of data transfer.
>
> Assuming radio activity of anywhere between 10 and 30 percent of
> operating time, Atheros calculates that the energy cost of
> sending/receiving a megabyte of data (at maximum data rate) is between
> four and nine times greater for 802.11b equipment than for 802.11a.
>
> Then there's the question of the cost of the hardware itself. Product
> manager Chen was quick to point out that as a chip supplier, Atheros
> does not control pricing at the end-user level. (Although the
> company's decision to use relatively inexpensive CMOS chip-making
> technology was consciously aimed at minimizing hardware costs.)
> Nonetheless, Chen said, the companies that will be selling products
> based on the Atheros chipset - which include Intel, Proxim, and TDK -
> have stated intentions to price their hardware "similarly" to 802.11b
> equipment. We won't have to wait long to find out what this means.
>
> Check this column tomorrow for more about 802.11a
>
> August 27, 2001
> --
> Robert J. Berger - UltraDevices, Inc.
> 257 Castro Street, Suite 223 Mt. View CA. 94041
> Voice: 650-237-0334 VoiceMail: 408-882-4755 Fax: 408-490-2868
> Email: rberger () ultradevices com  http://www.ultradevices.com



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